Known from prior art are various extrusion methods in which a plastic starting material is melted, mixed with optional additives and colorants and extruded through a die of a certain profile in order to form a product. When emerging from the die, the product is in, or close to, its final shape. From the die, the product can be led to a calibration unit, where it is provided accurate and final dimensions, and further to a water reservoir functioning as a cooler. The product is sawed into the desired size on the production line. The starting material is most often fed into the extruder in granulate or powder form.
Also known from prior art are different kinds of wood-plastic composite materials and methods for their production. Wood-plastic composites produced by the extrusion method are made by adding a wood-based constituent and different kinds of processing auxiliaries as fillers to the matrix plastic. The wood-based constituents used are typically sawdust but may also be wood chips or chemical pulp fibre. The starting materials are normally fed as granulates into the extrusion. The wood-based starting material as well as the plastic must therefore be first extruded in separate processes into granulates. Additional processing increases production costs. However, it is possible to feed the starting material components as such into the extruder and make them into products. This requires the use of different kinds of force feed devices to ensure a steady feed despite heterogeneity and low specific density of the starting material. The method is called direct feed.
When it is desirable that the wood-based constituent added to the matrix reinforces the profile, the fibre length and shape of the added constituent must be taken into account. The sawdust used in wood-plastic composites is so sized and shaped that it does not actually reinforce the product but instead functions merely as a filler in the plastic matrix.
Sawdust or wood chips normally require drying before the actual processing, because high moisture content in the filler impairs the quality of the manufactured product. The water vapour which is gasified in the process produces gas bubbles to the profile, which have effect on the look of the product as well as its mechanical properties. Drying, which is performed as an additional process, is an unwanted step due to additional costs, but also to the risk of explosion.
Known from prior art is also the manufacture of wood composites from wood waste arranged in the form of pellets, and from recycling plastic. Also known from publication U.S. Pat. No. 4,874,095 is the manufacture of wood composite materials from paper and thermoplastic material, and from publications U.S. 20020034629 and U.S. Pat. No. 3,718,536 the manufacture of a wood composite material from wood-based and plastic-based materials.